If I encounter a life form I don't recognise, a life form that doesn't have enough similarities with the earthling life form I know of:
I would first think it must be a mutant (quite advanced stage of mutation compared to natural evolution), and then maybe I would consider the possibility of alien life form
1- I would observe it while keeping a security distance of around 20 meters... Well, it depends on its size (here let's assume it's human sized), its members (does it look like it can run or does it crawl, or can it fly) where we are when we meet, wether it is on open field (40 meters minimun for security distance) or in a forest, or in an abandonned building
2- If the creature doesn't move, I would say "hello", or "hola, que tal", or "nihao", maybe, MAYBE, use some small hand gestures.
3-If the creature doesn't react to my words and (MAYBE) hand gestures, I would then use the UNIVERSAL LANGUAGE (if there is anything i can use to write while keeping my distance): PRIME NUMBERS!! I would write the 7 first primes numbers (I wouldn't write it like 2,3, 5, 7, I would write it like that: II III IIIII IIIIIII IIIIIIIIIII IIIIIIIIIIIII IIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIII).
If it react and give me at least the next 2 prime numbers, then I think I would have unlocked the achievment "First Communication"
If there is nothing to write, try using sound (like claping your hands) or light signals (with your phone??)
WARNING: Keep in mind that if you meet the alien when he just got out of his space ship, it would be best to start with prime numbers rather than speech and wild hand gestures. I think it's the best way to convey your wish for communication and lack of hostility (and the best way to avoid sounds/movments that may be considered hostile by the alien).
Also, the best way to express agreement or disagreement/interdiction is to use some basic equations (like 1+1=2 to say Yes, and 1+1=3 to say No), also, like I said for the prime numbers, write it in a manner that is easily understandable, don't write it in human language since the alien may not know it